The invention relates to a method for the production of cement clinkers low in alkali from alkali-containing raw material, whereby the latter is thermally treated in a multiple, step calcination process, preferably consisting of a pre-heating step, a deacidification step, a sintering step and a cooling step, and whereby fuel of any type is introduced both in the deacidification step as well as also in the sintering step, and hot air of the cooling step is supplied both to the calcination step as well as also to the sintering step as combustion air. The invention relates also to an apparatus for carrying out the method.
It is known that alkali-compounds in cement may appreciably shorten the solidification time and carry out the so-called change-over of the cement. Furthermore, it is known that too high an alkali-content in the cement may lead to blistering of alkali-sulfates in the concrete. Also, with the so-called additive substances capable of reaction, through a high alkali-content in the cement, a reaction of the alkalies with these additive substances may cause an alkali-expansion of the cement and thereupon endanger the constancy of volume of the concrete. Upon burning of cement clinkers, out of alkali-containing raw material, the alkalies, particularly the alkali-chlorides are volatilized quantitatively in the combustion furnace and reach with the exhaust gas of the combustion furnace into the preheater whereby they deposit themselves on the raw material and are conveyed back again in circulation with the pre-heated raw material into the combustion furnace. In this way, result the alkali-circulations in the stream of material and stream of gas, known to every expert, which lead to high increases in alkali and to cakings or deposits of alkali-compounds in the system of the calcination installation.
For the prevention of these disadvantages, it is known to burn the cement clinkers in two steps. According to earlier suggestions, the alkali-containing exhaust gases of the combustion furnace--sintering step--are conveyed off from the total calcination process, so that no alkalies may condense on the colder raw material. In this manner, no highly increasing circulation of harmful material can build up in the system. This method has, however, the extraordinarily great disadvantage that the sintering step in each case is lost without utilization of the combustion process, whereby the production costs of the cement are increased up to complete lack of economy.
In more recent times, for the production of cement clinkers low in alkali, from alkali-containing raw material, a method has become known (German Laid-Out Specification No. 22 62 213) in which a part of the exhaust gases of the sintering step is continuously removed through a bypass from the combustion process and the other part of the furnace exhaust gases is used for the raw material drying. For the pre-heating and calcination of the raw material, hot cooler-exhaust-air is used, in which additional fuel is burned, whereby the exhaust gases of the calcination step are used for the preheating of the raw material in the preheating step. The higher the alkali-content in the furnace exhaust gases, the higher is also the portion of the furnace gases which are discarded directly out of the system. In the case of less alkali-containing furnace exhaust gases, indeed a greater part of the exhaust gases are used for the raw material drying, however, in this connection, thermally high-valent furnace exhaust gases must indeed be directly discarded. This method can accordingly in no manner satisfy in respect of an economical heat-use of the combustion process. Besides, this method is unsuitable upon the treatment of raw material with greatly fluctuating and timewise very high alkali-content.